The present invention relates to an electronic balance in which a weight to be measured is counterbalanced automatically with a force produced by an electromagnetic force transducer.
Such an electronic balance is basically constituted with a self-equlibrating servomechanism consisting essentially of a weighing pan, a pan displacement detector for detecting the displacement of the weighing pan, a PID control system and an electromagnetic force transducer having its constituent movable force coil mechanically connected with the weighing pan. The PID control system contains a PID operation circuit, which outputs a PID control signal by computing and summing up a proportionality signal (P) proportional to an instantaneous value of, an integral signal (I) proportional to a time-integrated value of and a differetial signal (D) proportional to a differentiated value of the output from the tray displacement detector. In addition to the above basic constitution, the balance is provided with a pan stopper for mechanically restricting, when the balance is loaded with a large weight, the displacement of the weighing pan within a limited region. Without the pan stopper, a too large displacement of the weighing pan may cause the balance to give an erroneous measurement due to an elastic after-effect produced in the mechanism of the balance owing to a large distortion of the same.
However, the restriction on the movable range of the weighing pan causes a disadvantage that it takes a long time for the balance, when loaded with a large weight, to be brought into an equlibrium state. While the weighing pan has its displacement limited by the pan stopper, the output from the pan displacement detector remains at a value restricted by the pan stopper, irrespective of the magnitude of a weight placed on the weighing pan. Accordingly, because the PID operation circuit operates with an input signal smaller than that expected if the weighing pan is freely displaced meeting the weight placed thereon, it takes a long time for the electromagnetic force transducer to produce a force enough to compensate the large weight on the weighing pan. Such a disadvantage can be eliminated in principle by increasing the gain of the PID operation circuit only while the weighing tray continues to rest on the tray stopper. However, the mehtod of increasing the gain of the circuit is accompanied by a disadvantage that the balance is often caused to get into hunting or irregular flunctuations.